Helper activity in vitro to a defined determinant

To selectively induce a thymus‐derived (T) cell response to the hapten azophenylarsonate (Ars) we have injected BALB/c mice with the hapten covalently attached to mycobacteria. Spleen suspensions were then tested for evidence of Ars‐specific helper function by their ability to restore the in vitro anti‐lactoside (Lac) secondary response of T‐depleted Lac precursor cells. Helper activity was demonstrated when a double conjugate containing both Lac and Ars was added. Using column procedures which exclude adherent and Ig‐bearing cells, and treatment with anti‐T antiserum we excluded antibody as the responsible agent and implicated T cells. A comparison of the activity of hapten and protein‐specific helper cells revealed differences which may explain the infrequent demonstration of haptenspecific T function following immunization with hapten‐protein conjugates. The demonstration of Ars‐specific helper activity is interpreted as support for the contention that receptors on T cells are basically similar to those on antibody precursor cells.